This invention relates to cellular communications, and more particularly to a pattern of microcell base station antennae for use in dense urban areas.
Wireless mobile telecommunications systems utilize radio signals to exchange information between fixed-wire base stations and free-standing mobile stations. Each base station transmits from an antenna over a fixed-area range called a cell. In order to provide service to mobile stations over a large geographic area, many cells are positioned over the area to provide complete coverage.
In typical cellular configurations, base station antennae are located high above the ground, resulting in large, hexagonal or circular-shaped macrocells with the antenna located at the center of the cell. The transmissions from the base station propagate outward 360 degrees from the antenna. These cells are often referred to as "omni-directional."
Because the spectrum of radio frequencies allocated for telecommunications is limited, each cell operates on a different subset of frequencies to minimize the density of the used spectrum in a particular geographic area. To optimize the use of the allocated frequencies, cellular systems will reuse frequency sets from other cells. The reuse of frequencies is restricted, however, by co-channel interference from nearby cells which use the same frequencies. Accordingly, various cell layouts and patterns have been developed to maximize the distance between the closest cells operating on the same frequencies. The prior art shows that various distributions of frequencies may be made among cells depending on the relative location of each cell.
Using standard cell arrangements in dense urban areas presents some difficulties, however. The landscape of typical urban areas contains blocks of tall buildings separated by a grid-like pattern of streets. In these environments, the normally circular propagation patterns of base station antennae are blocked by the tall buildings. Accordingly, smaller cells called "microcells" are used in these environments. These smaller microcells are created by locating base stations and their antennae close to street level. The surrounding buildings are purposely used as propagation obstacles to prevent interference among neighboring microcells.
In contrast to the circular transmission propagation of ordinary cells, however, these urban microcells, which utilize buildings as propagation blockers, transmit only along streets which are within the line-of-sight of the base station antenna. Microcell patterns for these environments are thus designed differently from those of conventional macrocells. Like any cellular system, the allocated frequency band is limited, and reuse of the same frequencies at separate locations is critical. Furthermore, the ability of the microcells to reuse the same frequencies at a closer distance is often the reason for service providers to convert existing macrocells to microcells in urban or suburban areas to improve the system capacity. Reuse of the same frequencies generates co-channel interference which must be controlled to an acceptable level. In urban microcells, co-channel interference is dominated by interference from cells within the line-of-sight of another cell. Previous cell plans for these city environments were typically classified into "half-square", "full-square", or "rectangular" cell patterns. These cell patterns generally required a large number of microcells to cover an urban area, and did not take advantage of the full transmission range available to some base stations.